1. Field of the Invention
This invention relates to a surface treatment method by sputtering, more particularly, to a surface treatment method suitable for the continuous coating of fibers, strands, wires, bars, tubes and other elongated members.
2. Discussion of the Prior Art
A number of dry plating processes have been developed including vacuum deposition, ion plating and sputtering. As compared with film formation by conventional wet plating processes such as electroplating and electroless plating, the dry plating processes have many advantages that they can be applied to a wider variety of objects, coating can be made on a variety of materials, and coating which is unachievable with the wet processes, high functional coating and thin-film formation are possible. Because of these advantages, the dry plating process has quickly spread and is now a key technology covering almost all fields of industry including semiconductor, optoelectronics, recording media, electric appliances, automobiles, tools, and decorations.
The dry plating process suffers from several commercial problems in that the apparatus involved is expensive and complicated because the process has to use a vacuum system and the rate of film formation is not sufficiently high. It is particularly true when objects to be coated are fibers, wires, strands, bars, tubes and other elongated members. The percentage of coating material which is deposited on the outer surface of these elongated objects is lower than when objects to be coated have a generally planar surface. It is difficult to coat the outer surface of elongated objects to a uniform thickness. When it is intended to coat an elongated object at a speed as high as several tens of meters per minute, the apparatus must be greatly extended in the longitudinal direction of the object. The term elongated objects used herein include fibers, strands, wires, bars and tubes and similar members whose cross section may be either circular or non-circular and either solid or hollow.
Referring to FIG. 6, there is schematically illustrated a conventional sputtering apparatus. The sputtering apparatus generally designated as 1 delimits a vacuum chamber in which a target 2 and an object 3 to be coated are placed in an opposed relationship. The target 2 which is made cathode is connected to a DC or RF power source (not shown). With the power on, ions strike the target to drive atoms out of the target material. When the object 3 is a length of fibers, strands, wires, bars, tubes, or other elongated members, atoms of the target material are deposited on the surface portion of the object that is opposed to the target. Only a minor proportion of the atoms is deposited on the rear surface portion of the object, failing to accomplish uniform coating. It is necessary that once the front side of the object that is opposed to the target is coated, the object be rotated such that the rear side is opposed to the target and coating be carried out again, before an equal coating can be formed on the rear side of the object. In addition, the majority of target atoms or coating material is scattered in vain without being collected on the object 3 as shown in arrows in FIG. 6. The percent of target material actually coated is very low.
The same applies when the dry plating process used is vacuum deposition or ion plating.
There is a need for a method capable of dry plating fibers, strands, wires, bars, tubes and other elongated objects uniformly in an efficient and economical manner.
The only prior art known method meeting the requirement is a coaxial hollow sputtering method. A target is attached to the inside wall of a cylindrical vessel and made cathode. A magnetic field is applied from outside the vessel if necessary. Dry plating is carried out on an object disposed in the vessel. Plating can be carried out while the object is continuously moved in an axial direction. This method is satisfactory in uniformity of coating thickness and utilization of target material, but suffers from such disadvantages as troublesome replacement and manufacture of targets and increased cost.